2020欧洲杯APPWhen the S&P 500 closed at a record on Feb. 19, American stocks seemed untouched by the anxiety around the coronavirus epidemic that had already rattled investors in other markets, including bonds, commodities and some foreign shares.

Nine days later, an unnerving reality has set in. The S&P 500 suffered its fastest-ever 10% decline from an all-time high. The Dow Jones Industrial Average just had its worst week since the start of the 2008 global financial crisis. Shares of manufacturers, banks and utilities alike have...